1. Field of the Invention
The present invention relates to a print control system for a printing device which prints dot patterns by ejecting ink droplets from a print head while moving a carriage mounted with the print head and more particularly to improving print quality by rapidly changing duration of a print period, at which dots are printed, according to fluctuations in moving speed of the carriage.
2. Description of the Related Art
There have been known ink jet printers with a print head formed with a plurality of jet nozzles. The print head is mounted on a carriage reciprocally movable in a print direction while maintained in confrontation with a print sheet. By ejecting ink from nozzles while using a carriage drive motor to drive the carriage in the print direction, an image can be formed out of dot patterns on the print sheet.
An encoder and a photosensor for determining timing of dot printing are provided in the printer. Either a circular encoder plate formed with a plurality of radial slits is attached to the carriage drive motor or a band-shaped encoder plate formed with slits juxtaposed in a line is disposed parallel to the movement direction of the carriage. The photosensor includes a light emitting element and a light receiving element attached to transmit and receive light through the slits. The photosensor emits a pulse signal formed from alternating high and low level signals, which correspond to presence and absence of slits, and having a period equal to the sum of successive high and low level signals. The number of high and low level signals outputted from the photosensor is proportional to the distance moved by the carriage. Also, time duration of one period of the pulse signal indicates the speed of the carriage. By calculating these pulse signals, the print position of the carriage and the movement speed of the carriage can be determined so that timing of the dot printings can be determined.
Recently, the print resolution of ink jet printers has been increased, for example, to 360 dots per inch (DPI) or 720 DPI, in order to improve quality of images is printed by the ink jet printer on paper. There have been known some problems when the print timing for ejecting dots is determined by the timing of rising and falling edges of the pulse signal outputted by the photosensor. For example, when the plate-shaped encoder is incorrectly attached so that slits and slats are shifted out of correct alignment, then the time duration of high and low level signals can vary. Also, duration of one period can vary because of fluctuations in the rotational speed of the carriage drive motor. In addition to these problem, when the print resolution is 360 DPI or 720 DPI, the slits must be formed in the encoder plates at an extremely narrow pitch, which makes their manufacture virtually impossible or prohibitively expensive. Also, paper dust or other debris can partially or totally cover slits, thereby altering the detected period.
Japanese Patent Application (Kokai) HEI-6-66666 describes a pulse interval conversion unit for an ink jet printer with two encoders for detecting each slit in a slit plate attached to the carriage drive motor. Each of the encoders outputs a pulse signal, one signal having an A phase and the other having a B phase, the A phase and the B phase having a phase difference. The ink jet print printer is also provided with a pulse interval calculation circuit, a digital filter, a standard pulse generation circuit, and a pulse interval conversion circuit in a configuration for determining the duration of one period by adding four separate pulse time durations together. The four time durations added together to determine the period are the pulse time duration from the rising edge of the A phase to the rising edge of the B phase, the next pulse time duration from the rising edge of the B phase to the falling edge of the A phase, the next pulse time duration from the falling edge of the A phase to the falling edge of the B phase, and in the next pulse time duration from the falling edge of the B phase to the rising edge of the A phase.
The calculated duration for one period is divided by a predetermined number of four to determine an average pulse time duration. A print timing pulse is generated each time the average pulse time duration elapses. The print timing pulses serve as a reference pulse train.
Additionally, reference pulse number values and delay coefficient values are prestored in correspondence in a change table in accordance with different ratios of change in the pulse interval between adjacent pulses. A reference pulse number value and a delay coefficient value for a selected pulse interval ratio of change are retrieved from the change table. The delay coefficient value is multiplied by speed data to determine a pulse interval. Each time a duration of time corresponding to the calculated pulse interval elapses, a print timing pulse is outputted in association with a reference pulse until a number of reference pulses has been applied. As a result, each print timing can be determined based on rotational speed of the carriage drive motor.
Japanese Patent Application (Kokai) HEI-2-28163 discloses a drive amount detection device which determines, based on two pulse signals having A and B phases respectively, print timing for dot printing by determining duration of one period by adding four pulse time durations.